1. Field of the Invention
The present invention relates to complex oxide particles predominantly containing zinc oxide, whose primary particles exhibit a low degree of aggregation, and which are easily dispersed suspended, or kneaded, with involvement of no pulverization or slight pulverization, in non-aqueous solvents, aqueous solvents, or organic polymer compositions such as resin; to a process for producing the particles; and to applications of the particles.
2. Description of the Related Art
Zinc oxide, also called zinc flower, has long been known as a white pigment. Zinc oxide, zinc oxide doped with a different element, and zinc oxide which has undergone surface treatment such as coating (hereinafter these types of zinc oxide may be collectively referred to as “zinc oxides”) exhibit the following optical properties when such zinc oxide is formed into fine particles having a diameter approximately half the wavelength of visible light, the scattering effect of the zinc dioxide particles is reduced considerably. As a result, the particles allow visible light to pass therethrough, and selectively absorb ultraviolet rays by virtue of excellent ultraviolet absorbing effect of zinc oxide.
Conventionally known ultraviolet absorbers include organic ultraviolet absorbers such as benzophenone-based absorbers, benzotriazole-based absorbers, salicylate-based absorbers, and substituted-acrylonitrile-based absorbers. However, a limitation is imposed on incorporation of such an ultraviolet absorber into cosmetics or pharmaceutical products, from the viewpoint of safety. Meanwhile, when such an ultraviolet absorber is incorporated into a thermoplastic resin or a similar material and the resultant mixture is subjected to molding, since the absorber exhibits poor heat resistance, the absorber is decomposed or undergoes bleedout during molding.
Therefore, zinc oxide exhibiting excellent safety and high heat resistance has become of interest as an ultraviolet absorber. Such zinc oxide is incorporated into cosmetics or pharmaceutical products, or is employed as a filler in, for example, resin films or organic polymer compositions.
Most cosmetics contain organic substances such as oil, fat, wax, and an organic ultraviolet shielding agent; inorganic powder such as a coloring pigment or an extender pigment; and zinc oxides capable of shielding ultraviolet rays. Such zinc oxides must exert excellent ultraviolet shielding effect. In addition, the zinc oxides must exhibit excellent transmission of visible light while maintaining transparency, without causing the skin to be unnaturally highlighted. Furthermore, the zinc oxides must provide a good sensation upon use; i.e., smoothness or slidability, and must exhibit excellent dispersibility when incorporated into other materials.
When zinc oxides are employed as a filler in, for example, resin films or organic polymer compositions, particularly in food packaging film, freshness maintaining film, product display film, or agricultural film, transparency of a medium must be maintained.
Zinc oxides satisfying the aforementioned requirements must assume the form of fine particles, and must exhibit good dispersibility in a medium. Numerous techniques for obtaining such zinc oxides have been proposed.
Japanese Patent Publication (kokoku) No. 60-5529 discloses a method for producing zinc oxide fine particles having a size of about 0.2 to 1 μm, in which zinc assuming a gaseous form (hereinafter may be referred to as “zinc vapor”) is oxidized and combusted, and then immediately cooled at a cooling rate of at least 480° C./second. At a temperature of 350° C. or higher, large zinc oxide particles are formed, and the specific surface area thereof is reduced. Therefore, cooling is carried out at such a high rate, to thereby prevent formation of large particles. However, when cooling is carried out at a high rate, oxidation of zinc becomes insufficient, and gray zinc oxide is formed. When such gray zinc oxide is employed in cosmetics, etc., sufficient transparency cannot be obtained. Since production of white zinc oxides requires complete oxidation, sufficient oxidation time must be provided. Therefore, a limitation is imposed on the micronization of zinc oxides.
When zinc oxide is irradiated with ultraviolet rays, the zinc oxide exhibits photocatalytic activity; i.e., excited electrons and generated holes cause various oxidation and reduction reactions. Due to the photocatalytic activity, the zinc oxide decomposes an organic substance which is in contact therewith. Therefore, when zinc oxide whose photocatalytic activity is not suppressed is employed in cosmetics, stability of the cosmetic is lowered. Meanwhile, when such zinc oxide is employed in a filler or a film, an organic substance contained therein is impaired, imparting poor weather resistance to the filler or film.
When zinc oxide is employed in cosmetics or resins, the surface activity of the zinc oxide must be reduced through, for example, surface treatment.
With regard to the surface treatment method, Japanese Patent Application Laid-Open (kokai) No. 3-183620, for example, discloses a method in which zinc oxide fine particles are added to a sodium silicate aqueous solution and stirred and the resultant mixture is subjected to pH adjustment, to thereby form an Si oxide on the surface of the zinc oxide. In the method, reaction is allowed to proceed in a liquid phase, and therefore a solid-liquid separation process and a powder drying process are required. Therefore, aggregation of the resultant powder is inevitable, and dispersibility of the powder is lowered. When the mixture is concentrated and employed in the form of slurry without solid-liquid separation, dispersibility of the slurry is improved as compared with that of the powder, but a great limitation is imposed on the degree of freedom in terms of formulation.
Japanese Patent Application Laid-Open (kokai) No. 2001-558821 discloses a method for preventing photocatalytic activity of zinc oxides by coating the oxides with zinc silicate. However, in the method, reaction is allowed to proceed in a liquid phase, and therefore aggregation of the resultant powder of zinc oxides is inevitable.
Japanese Patent Application Laid-Open (kokai) No. 5-319808 discloses a method in which a metal oxide nucleus is formed through heating, gasification, and thermal decomposition of an organo-metallic salt, and then another type of organo-metallic salt is thermally decomposed on the nucleus, to thereby form a coating layer. In the method, a nucleus formation process and a coating layer formation process must be separated. Since conditions for the nucleus formation process are different from those for the coating layer formation process, control of these processes is difficult, and thus productivity is poor, resulting in high production cost.
As similar techniques, Japanese Patent Application Laid-Open (kokai) Nos. 6-144834 and 6-144833 disclose a method for producing electrically conductive zinc oxide. However, since the amount of a dopant is very small, the photocatalytic activity of zinc oxides is not necessarily prevented.
There have been proposed resin additives or cosmetics containing zinc oxide which is not subjected to surface treatment.
For example. Japanese Patent Application Laid-Open (kokai) No. 7-89710 discloses a method for producing a composition containing uniformly dispersed zinc oxide and silicic acid anhydride, in which a mixture of a sodium silicate aqueous solution and zinc oxide is added to a solution mixture of a surfactant and an organic solvent, a calcium chloride aqueous solution is added to the resultant mixture, and the thus-obtained mixture is subjected to pH adjustment. However, the composition produced through the method contains a small amount of zinc oxide, assumes the form of highly aggregated powder, and exhibits poor ultraviolet shielding and poor transmission of visible light transmission.
Japanese Patent Application Laid-Open (kokai) No. 7-118133 discloses a cosmetic composition containing zinc oxide fine particles, which exhibits excellent transparency, ensures long-term make-up quality because of its high compatibility with free fatty acids contained in sebum, and has excellent ultraviolet shielding property. However, since the cosmetic composition contains zinc oxide having surface activity and an organic substance, decomposition of the organic substance is inevitable, and as a result, stability of the cosmetic composition is impaired.
Japanese Patent Application Laid-Open (kokai) No. 7-25614 discloses zinc oxide exhibiting excellent transmission of visible light and excellent ultraviolet shielding. However, the zinc oxide disclosed in this publication is similar to the zinc oxide disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-118133 in terms of insufficient suppression of the photocatalytic activity of zinc oxide.
As described above, although zinc oxide fine particles obtained through conventional techniques have ultraviolet shielding property, the particles are not easily dispersed in a medium, due to their aggregation. In addition, the particles exhibit insufficient transmittance of visible light; i.e., the particles exhibit low transparency.